P-glycoprotein has been associated with a multidrug-resistant (MDR) phenotype in a wide variety of animal and human tumor cell lines and clinical tumor specimens. Pediatric tumors, however, have not been included in the studies reported so far. Given the known chemosensitivity of certain solid pediatric tumors, such as Ewing's sarcoma, primitive neuroectodermal tumor and rhabdomyosarcoma, to the point that all clinical trials use chemotherapy and radiotherapy as primary methods of treatment, it is important to detect levels of P-glycoprotein in these tumors before and after treatment. Tissues from Ewing's sarcomas, primitive neuroectodermal tumors and rhabdomyosarcomas are available from patients included in the clinical therapeutic protocols in the Pediatric oncology Branch of NIH. All these tumors are well characterized and classified histologically by light and electron microscopy. Moreover, tissues from primary sites before treatment and from metastatic sites or sites of recurrence after treatment are available in many cases. All these tissues will be evaluated for the presence of P-glycoprotein using a commercially available antibody against P-glycoprotein and employing a modified immunoperoxidase "sandwich" staining method (Chan et al., Lab. Invest. 59: 870, 1988). The results of immunostaining for P-glycoprotein will be correlated with clinical data of patient outcome and response to treatment.